The present invention relates to the art of coal production, and, in particular, to the art of controlling dust and spontaneous combustion resulting from production, drying, handling, and transportation of coal.
Dust, which is produced in large quantities as a result of handling, transporting and storage of coal, is a constant source of annoyance and danger, as it is the cause of coal dust explosions and because it presents a hazard to the respiratory system. Coal dusting is also the cause of significant coal loss during transportation.
This dust is a result of degradation caused partly by the escape of moisture from the coal and partly by the oxidation of mineral matter, particularly iron sulfide, contained in cleavage veins of coals and similar fuels. Another cause for dust production, which occurs in both high rank and low rank coals, is a high degree of mechanical abrasion which necessarily results during handling and transportation.
Accordingly, since coal remains an important chemical and energy source, concentrated efforts to control dust must be made in order to insure its commercial availability.
For example, in the case of low rank coals, subbituminous, lignite and peat, the moisture content is between about 20 to 50 wt % or higher, resulting in low heat content. Much of the low heat content coal mines in the United States are generally located in areas with low demand for coal, for instance, Wyoming and North Dakota, and this coal is expensive to transport so that the marketing is severely restricted. Moreover, high moisture coal can freeze in the winter and make handling and transportation difficult. In addition, use of low heat content coal in an industrial furnace often requires derating of the existing units and increasing design capacity for the new units. Thus, the incentives for drying such low rank coal are obvious. Many companies, both coal producers and drying equipment manufacturers are developing techniques for drying low rank coals.
In comparison with Eastern high rank coals, the Western low rank coal is more difficult to dry. The desirable moisture level of the dried product is generally accepted to be about 8-12 wt % which is lower than the surface moisture level. Surface moisture, defined as the water held loosely by coal which can be removed easily by physical means such as draining and centrifuging, etc., is about 3-10 percent by weight. As a result, the Western coal has to be dried at more severe conditions and higher temperatures. Under such conditions, coal particles tend to shrink, crumble and disintegrate to form excess amounts of fines and dust. Excessive amounts of fines and dust degrade the product and make the product difficult to handle and transport, and most importantly, cause safety and fire hazards.
Dried low rank coal tends to combust spontaneously during transportation and storage. The dusting problem is particularly serious when a fluidized bed dryer is used, and, in recent years, the new dryers are almost entirely fluidized bed types which are schematically shown and discussed in Coal Age 109, January 1976, and Coal Mining & Processing, 39 October 1969.
Thus, in order to understand more about coal dust formation, studies have been conducted of the fragmentation pattern of subbituminous and bituminous coal as a function of feed particle size.
It has been generally known in the art of allaying dusting in coal to use oils or aqueous mixtures of deliquescent chemicals such as calcium chloride, magnesium chloride, and potassium carbonate. See generally U.S. Pat. Nos. 2,319,942; 2,448,605; 2,786,815; 3,533,953; 4,038,443 (which also shows the use of clay in aqueous solution); and U.S. Pat. No. 4,067,818. However, with the continuing rise in petroleum prices, the use of petroleum based dust control agents is becoming or has become economically unattractive for large scale use. In U.S. Pat. No. 4,269,721, the use of a calcium sulfate-containing composition, which is produced from the fluid bed combustion of coal, is disclosed as a means for preventing dusting. All of the agents listed above are believed to be rather expensive and/or tedious in use and/or introduce pollutants to the atmosphere during burning and thermal operations performed on coke or coal.
More recently, efforts to control dust have resulted in the use of an aqueous solution containing cellulose ether along with a wetting agent applied to stockpiles of aggregate materials at a rate of from about 0.05 to about 0.20 gallons per square foot of surface area. See U.S. Pat. No. 4,369,121. Further attempts to effect dust control have led to development of a liquid dispersion of watery consistency of highly branched water swellable polymer of acrylamide or an acrylamide-acrylic acid copolymer, branched and/or crosslinked with a multifunctional unsaturated monomer containing more than one ethylenically unsaturated group. The crosslinked polymer or copolymer has a three-dimensional structure and is in the form of microgelatinous particles having a particle size not exceeding approximately one micron in an oil containing emulsion form and having the property of swelling dramatically in water and binding dust particles upon adsorption from solution. See U.S. Pat. No. 4,417,992. See also U.S. Pat. No. 4,316,817 and U.S. Pat. No. 4,136,050. These methods, however, involve a rather elaborate approach and somewhat careful preparation and are not believed to provide a simple inexpensive method of controlling dust.
U.S. Pat. No. 4,055,471 to Beck et al. discloses the use of an aqueous solution of 30-70% by weight of waste sulfite liquor as a means for inhibiting dust formation while feeding coal into a coking chamber. Alternative binders considered suitable for controlling coal dust include starches, dextrin, molasses, casein and glue. Surfactants used by Beck et al. to aid in the wetting of the hot coal include sodium soap, potassium soap, sulfate of fat alcohols and fat alcoholpolyoxyethylene products. There is no indication or suggestion by Beck et al., however, with regard to providing a dust inhibiting composition for controlling coal dust during transportation.
It is, therefore, an object of the present invention to provide a simple, inexpensive means of preventing and/or reducing the incidence of dust by use of naturally occurring derivative products, especially in relationship to the production and handling of low rank coal.